1. Field of the Invention
The present invention relates generally to an image information processing apparatus. More particularly, this invention relates to an image information processing apparatus that separates an image signal into a luminance component and a chrominance component and produces luminance data and chrominance data using a single phase-locked loop.
2. Description of the Related Art
An image signal for reproducing a color image on a monitor screen includes a luminance component, a chrominance component and a sync component. The luminance component represents the tone of an image. The chrominance component expresses color. The sync component represents information about various types of synchronizations, such as horizontal synchronization and vertical synchronization. Those components are separated from one another by using characteristics like the difference in amplitude of each components and phase deviation, which yields a luminance signal, a chrominance signal and a sync signal. Image processing is performed using those signals. Digital signal processing, which is not easily susceptible to a temperature variance, noise, etc. is likely to be used in this image processing.
FIG. 1 is a block diagram showing an image information processing apparatus which is based on digital signal processing. FIG. 2 is a waveform chart illustrating signals that are handled in the apparatus in FIG. 1.
The image information processing apparatus includes a Y/C separator 1, first and second amplifiers 2 and 4, first and second A/D converters 3 and 5, a sync detector 6, first and second phase-locked loops (PLLs) 7 and 9, a burst detector 8 and an image data processor 10.
The Y/C separator 1 receives an image signal "i" and separates the image signal to a luminance component and a chrominance component using the phase difference of the chrominance component, thereby yielding a luminance signal "y" and a chrominance signal "c". In the case of the NTSC form, for example, the luminance component is acquired by adding an image signal, which is the image signal "i" shifted by one horizontal scan period, to the image signal "i". The chrominance component is acquired from the difference between the image signal "i" and the shifted image signal.
The first amplifier 2 is a high-frequency amplifier configured to cope with the video signal band that corresponds to the luminance signal "y" and amplifies the luminance signal "y" to have a predetermined amplitude. The first A/D converter 3 receives the amplified luminance signal "y" from the amplifier 2 and quantizes the amplified luminance signal "y" In accordance with a first sampling clock signal SC1 to produce luminance data Y0.
The second amplifier 4 is a high-frequency amplifier configured to cope with the chrominance signal "c" and amplifies the chrominance signal "c" to have a predetermined amplitude. The second A/D converter 5 receives the amplified chrominance signal "c" from the second amplifier 4 and quantizes the amplified chrominance signal "c" in accordance with a second sampling clock signal SC2 to produce chrominance data C0.
The sync detector 6 receives the image signal "i" and obtains a sync component from the image signal "i". The detector 6 also produces a horizontal sync signal HD and a vertical sync signal VD from the acquired sync component, In this sync detection, the mixture ot the horizontal sync component and vertical sync component is acquired first using the difference between the amplitude of the sync component and the amplitudes of the other signal components. Then, the mixed component is separated into the horizontal sync component and vertical sync component using the difference in frequency between those components.
The first PLL 7 receives the horizontal sync signal HD from the sync detector 6 and produces the first sampling clock signal SC1 for the luminance signal "y" based on the horizontal sync signal HD. In the NTSC form, for example, the first PLL 7 is configured so that a clock signal 25 obtained by frequency-dividing the first sampling clock signal SC1 by 910 synchronizes with the horizontal sync signal HD. Accordingly, the first A/D converter 3 produces 910 pieces of luminance data within one horizontal scan period.
The burst detector 8 receives the image signal "i" and selectively extracts a burst signal CB, as shown in FIG. 2, from the image signal "i" for phase synchronization of the chrominance component. The burst signal CB has a fixed, predetermined frequency (e.g., 3.58 MHZ). The burst signal CB is previously superimposed on the image signal "i" at a predetermined position of the image signal "i", e.g., at the back porch of each horizontal blanking period. The burst signal CB is therefore selectively extracted from the image signal "i" at the beginning of each horizontal scan period.
The second PLL 9 receives the burst signal CB from the burst detector 8 and produces the second sampling clock signal SC2 for the chrominance signal c based on the burst signal CB. The second sampling clock signal SC2 has the sane period as the first sampling clock signal SC1. In the NTSC form, for example, the second PLL 9 is configured so that a clock signal obtained by frequency-dividing the second sampling clock signal SC2 by 4 synchronizes with the burst signal CB. Accordingly, the second sampling clock signal SC2 having a frequency of 14.32 MHZ is produced on the basis of the burst signal CB having a frequency of 3.58 MHZ.
The image data processor 10 receives the luminance data Y0 and chrominance data C0 respectively from the first and second A/D converters 3 and 5, piece by piece, and performs predetermined signal processing to produce new luminance data Y and color difference data U and V. The color difference data U and V respectively represent the differences between the luminance component and the red and blue components.
The process of producing the luminance data "Y" involves processes such as an aperture process and a gamma compensation process. The aperture process is to emphasize the contrast of an image. The gamma compensation is to compensate a visual non-linearity with respect to the luminance level. The process of producing the color difference data U-and V involves processes, such as demodulation of the chrominance component, which has previously undergone balanced modulation, and white balance adjustment. Thereafter, the subtraction between the individual chrominance components and the luminance component is performed.
The image data processor 10 is configured to sample again one of the luminance data Y0 and the chrominance data C0 in consideration of the difference between the output timing for the former data Y0 and the output timing for the latter data C0. This structure compensates for the difference between the output timings for the luminance data Y0 and chrominance data C0. Therefore, this processor 10 supplies the luminance data Y and color difference data U and V to the recording system or the reproducing system at the same timing.
From the viewpoint of the operational characteristic of a PLL, it is difficult to use a single PLL for both the. first and second PLLs 7 and 9, which are respectively provided at the subsequent stages of the sync detector 6 and the burst detector 8. The luminance component is superimposed on the image signal "i" based on the horizontal sync signal HD, and the chrominance component is superimposed on the image signal "i" based on the burst signal. For the ordinary image signal "i", the horizontal sync signal HD and the burst signal CB scarcely synchronize with each other. This is because the period of the horizontal sync signal HD greatly varies due to the influence of a jitter included in the image signal "i", while the period of the burst signal CB, which has a fixed pattern, does not change significantly. The quantization of the luminance signal "y" and the chrominance signal "c" using the same sampling clock signal causes a deviation between the luminance data and color difference data due to the phase difference between the horizontal sync signal HD and the burst signal CB. In digital signal processing, particularly, the deviation between the luminance data "Y" and color difference data U and V clearly appears as the disturbance of the outline of an image or oozing color on the reproduced image. The use of the mutually independent first and second PLLs 7 and 9, however, enlarges the circuit area and results in increased costs.
Another disadvantage lies in that the mutually independent first and second PLLs 7 and 9 serve as two kinds of oscillation sources to oscillate frequencies close to each other. The oscillation of those two kinds of oscillation sources causes swell noise to be generated relatively easily. This low-frequency beat noise is likely to appear on the reproduced image. In other words, although digital signal processing provides a noise immune environment, noise is generated due to the quantization, or sampling.